Plagued by persistent allergy/sinus headaches in the afternoons, I tried this a few years ago and it is like a miracle.
I keep waiting for the "placebo effect" to wear off, but years later it is still working well for me for congestion, sinus pain.
For colds & flu, it MAY decrease your odds of catching if you take during the season but hard to say. I've caught less colds I was exposed to and have had less severe ones, but that also happens as you get older so might not be the preventive Kold Kare dosing.
Elderberry (tablets, liquid - brand name Sambucol is good) also works really well when you're having more runny nose/throat issues more than congestion/sinus. For me at least.

I have to take a number of medications daily, and always felt kind of "yucky" from the various side effects. I started taking silymarin figuring if it is good for a hangover, it would be good for me.
Within a week of taking it, I felt a lot better and more "clear" physically. When I stop, I quickly get an increase in side effects from my other medications.
I do think it is good to take a break from this herb every so often just in case it is making your liver do too much overtime / so you don't become insensitive to it.

Hard to say for sure, but really seems to help me when I'm sick. Also seemed to make my cough better.
There is a liquid form that is really good, probably easier for kids, and seems to slightly coat the throat which is soothing if sore.